Following subcutaneous injection, radiolabeled monoclonal antibodies bind efficiently to normal and tumor target cells in the lymph nodes. This finding prompted us to attempt specific therapy using monoclonal antibodies covalently coupled to plant toxins. The first development along these lines has been to synthesize monoclonal antibody-ricin A-chain conjugates using four antibodies of different specificities. We then demonstrated the capacity of these conjugates to bind to their target cells and to inhibit protein synthesis at the ribosomal level in an acellular system. Unfortunately, using the guinea pig hepatocarcinoma cell line (L 10), which expresses large quantities of target antigen, we found only weak cytotoxicity with the monoclonal antibody D3 coupled to ricin A-chain. However, the same toxin coupled to an anti-mouse MHC antibody has proved to be highly toxic for lymphoid cells and similar results have been found with ricin A-chain conjugated to nonclonal antibodies against subsets of mouse lymphocytes in vitro. We have also been able to augment the effects of these toxins by the action of certain drugs which are known to affect cell biological processes. Another approach to specific therapy within the lymphatic system is the subcutaneous injection of a monoclonal antibody followed by a similar injection of complement. Such a system attempts to reproduce physiological antibody/complement dependent cytotoxicity. The determination of optimal doses and injection regimes will be facilitated by our current studies on monoclonal antibody pharmacokinetics and by in vitro cytotoxicity assays. A further modality for specific cell killing in vivo is to use radioactive compounds attached to antibodies which, on binding to cell surfaces, can damage the cells by radiation. We have been able to demonstrate that lymph node cell ablation occurs in mice injected subcutaneously with an anti-murine B cell antibody labelled with the alpha particle emitter bismuth 212. Studies are in progress to optimize this system and to reduce the non-specific toxicity.